Hybrid density functional study of the p-benzosemiquinone anion radical: The influence of hydrogen bonding on geometry and hyperfine couplings

Hybrid density functional calculations (B3LYP) are performed on the p-benzosemiquinone anion radical in its free and hydrogen-bonded forms. Geometries and hyperfine couplings are reported. A variety of basis sets ranging from split valence to full triple-zeta are employed. Converged results for hyperfine couplings are observed at the double-zeta level. Hydrogen bonding principally leads to increased spin density on the carbonyl carbon leading to an increase in the C-13 isotropic and anisotropic hyperfine coupling of this atom. Comparison with experimentally determined isotropic and anisotropic hyperfine couplings shows good quantitative agreement between theoretical calculation and experiment.